Optical disk apparatus

ABSTRACT

The quantity of spherical aberration correction is predetermined for each recording plane of an optical disk on which focusing control is performed. Before operating the focusing control, the quantity of spherical aberration to be corrected by an aberration correcting system is set based on an output signal from an aberration correction quantity switching device that corresponds to the type of the optical disk and the recording plane to be subjected to the focusing control. This makes it possible to perform stable focusing control on each recording plane of a multi-layer optical disk with high density by using an objecting lens having a large NA after spherical aberration has been corrected properly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk apparatus forrecording, reproducing or erasing information on an optical disk thatacts as an optical information medium.

2. Description of the Related Art

The optical storage technology that employs a optical disk with pitpatterns as a high-density, large-capacity recording medium has been putinto practical use while expanding its applications to digital versatiledisks (DVD), video disks, document file disks, and data files. Thefunctions required for recording/reproducing information successfullyand with high reliability on an optical disk by a finely focused lightbeam (e.g., with a diameter of 1 μm or less) are classified into threemajor categories: a focusing function for forming a diffraction-limitedtiny spot, focusing control (focus servo) and tracking control functionsof an optical system, and a pit signal (information signal) detectingfunction.

To improve the recording density of an optical disk further, an increasein the numerical aperture (NA) of an objective lens has been studiedrecently. The objective lens focuses a light beam on the optical disk toform a diffraction-limited tiny spot. However, spherical aberration,which is caused by an error in thickness of a base material forprotecting a recording layer of the optical disk, is proportional to thefourth power of NA. Therefore, when NA is increased, e.g., to 0.8 or0.85, the spherical aberration becomes significantly large. Thus, ameans for correcting the spherical aberration is essential to theoptical system. FIG. 13 shows an example of such an optical system.

Referring to an optical pickup 11 in FIG. 13, numeral 1 denotes aradiation source such as a laser source. Alight beam 20 (a laser beam)emitted from the laser source 1 is converted into parallel light by acollimator lens 3, passes through a liquid crystal aberration correctingelement 4, and enters an objective lens 5 to be focused onto aninformation recording plane of an optical disk 6. The light beamreflected from the optical disk 6 retraces the same optical path so asto be condensed by the collimator lens 3 and is directed intophotodetectors 9, 10 by a light separating device such as a diffractionelement 2. Servo signals (i.e., a focus error signal and a trackingerror signal) and information signals are generated from output signalsof the photodetectors 9, 10. Here, the NA of the objective lens 5 is aslarge as 0.8 or more. An actuator 7 includes a driving means, such ascoils and magnets, and performs focusing control for positioning theobjective lens 5 in the direction parallel to an optical axis andtracking control for positioning it in the direction perpendicular tothe optical axis.

A transparent base material (not shown) is formed on the informationrecording plane of the optical disk 6 on the side of the objective lens5 and serves to protect information. Since differences in thickness andrefractive index of the transparent base material cause sphericalaberration, the liquid crystal aberration correcting element 4 correctsa wavefront of the light beam to provide optimum reproduction signals.The liquid crystal aberration correcting element 4 has a transparentelectrode pattern made of indium-tin-oxide (ITO) alloy or the like. Thein-plane refractive index distribution of the liquid crystal aberrationcorrecting element 4 is controlled by applying a voltage to thetransparent electrode so as to modulate the wavefront of the light beam.

FIG. 14 shows an optical disk apparatus 116. Referring to FIG. 14,numeral 8 denotes an aberration correcting element driving circuit 8that applies a voltage to the liquid crystal aberration correctingelement 4, and 118 denotes a control circuit that receives a signal fromthe optical pickup 11 and controls and drives the actuator 7, theaberration correcting element driving circuit 8, and the laser source 1.The control circuit 118 causes the laser source 1 to emit a light beamand controls the position of the objective lens 5 based on the signalfrom the optical pickup 11. Moreover, it drives the aberrationcorrecting element driving circuit 8 to improve information signals fromthe optical pickup 11.

In addition to the above example, JP 2000-131603 A also discloses anoptical system for the optical pickup 11, which is illustrated in FIG.15.

FIG. 15 shows the components of the optical system other than a lasersource, a collimator lens, and a photodetector. A light beam that hasbeen converted into parallel light by a collimator lens passes throughan aberration correcting lens group 201 and is focused on an opticaldisk 6 by an objective lens group 202. The aberration correcting lensgroup 201 includes a negative lens group 21 and a positive lens group22. The objective lens group includes an objective lens 302 and aforward lens 301. The space between the negative and positive lensgroups 21, 22 is changed to correct spherical aberration in the entireoptical system. To change the space between the two lens groups, e.g., adriving portion 25 that shifts the negative lens group 21 in the opticalaxis direction can be used. The driving portion 25 may be formed, e.g.,of a voice coil, a piezoelectric element, an ultrasonic motor, a screwfeeder, or the like.

In the above configuration, spherical aberration is corrected so as toimprove the quality of information signals on the assumption that theoptical disk 6 has a single information recording plane and focusingcontrol is performed stably on the information recording plane. For theDVD standard that uses an objective lens having an NA of 0.6, atwo-layer disk with two information recording planes is employed.Therefore, the two-layer disk structure as well as a larger NA iseffective in increasing recording capacity per optical disk.

As shown in FIG. 16, two-layer disk 6 includes a base material 62, an L0layer (a first recording layer) 63, an intermediate layer 65, an L1layer (a second recording layer) 64, and a protective layer 66 to formthe back of the optical disk, which are stacked in this order from theoptical pickup side. The base material 62 and the intermediate layer 65are transparent media of resin or the like. Since the intermediate layer65 is between the L0 layer 63 and the L1 layer 64, the thicknessmeasured from the surface 61 of the optical disk 6 on the optical pickupside to the second recording layer (L1 layer) 64 is larger than that tothe first recording layer (L0 layer) 63 by the thickness of theintermediate layer 65. Such a difference in thickness causes sphericalaberration. However, the magnitude of the spherical aberration can betolerated by the optical system of the DVD standard that includes anobjective lens having an NA of 0.6. Therefore, it is possible torecord/reproduce information without correcting the sphericalaberration.

When NA is increased to 0.8 or more so as to achieve a furtherimprovement in the recording density of an optical disk, sphericalaberration caused by the thickness of the intermediate layer 65 cannotbe ignored. In other words, the correction of spherical aberration isindispensable for recording/reproducing information on both of therecording layers. As described above, increasing NA to 0.8 or morerequires a means for correcting spherical aberration even if informationis recorded/reproduced on a single recording layer. Thus, as a matter ofcourse, it is necessary to correct spherical aberration optimally ateach of the recording layers when information is recorded/reproduced ona two-layer disk as shown in FIG. 16. This can eliminate the sphericalaberration caused by the thickness of the intermediate layer.

JP 10(1998)-188301 A discloses the correction of spherical aberrationthat is performed before operating focusing control on an informationrecording plane. FIG. 17 shows this configuration. An objective lens 302is held by a holder 305, and a forward lens 301 is held on the holder305 via a second drive means 304. Therefore, a first drive means 303that supports the holder 305 drives both of the forward lens 301 and theobjective lens 302 in a focusing direction. The second drive means 304drives the forward lens 301 relative to the objective lens 302 in thefocusing direction. The space between the forward lens 301 and theobjective lens 302 can be changed by driving the forward lens 301 in thefocusing direction with the second drive means 304, thus correctingspherical aberration.

In this configuration, the first drive means 303 drives the forward lens301 and the objective lens 302 together in the focusing direction.Therefore, these lenses are prone to deviate from the center and tilt,which makes it difficult to satisfy the strict tolerance of positioningaccuracy for the lenses 301, 302.

Next, the problems of the aberration correcting lens group including twolens groups, i.e., the positive lens group and the negative lens group,will be described. FIGS. 18A and 18B are schematic views showing theaberration correcting lens groups located with their optical axesextending in a horizontal direction and in a vertical direction,respectively.

The aberration correcting lens group 201 located with its optical axis201 a horizontal is explained by referring to FIG. 18A. As shown in FIG.18A, the positive lens group 22 is fixed and held by a stationaryportion 26, while the negative lens group 21 is held by a lens holder24. The lens holder 24 is held by the stationary portion 26 via aplurality of elastic wires 27. Therefore, the negative lens group 21 isheld by the stationary portion 26 in a cantilever supporting fashion. Adriving portion (not shown) that shifts the negative lens group 21 heldby the lens holder 24 in the direction of the optical axis 201 a isprovided to change the space between the positive and negative lensgroups 22, 21, thus correcting spherical aberration.

When the aberration correcting lens group 201 is located with itsoptical axis 201 a horizontal, there is no problem because the negativelens group 21 is at the position Y0 in the direction of the optical axis201 a as designed, and the space between the positive and negative lensgroups 22, 21 also is kept at a designed value A.

Depending on the orientation of an optical disk apparatus or the designof an optical pickup, the aberration correcting lens group 201 may belocated with its optical axis 201 a vertical. This configuration isexplained by referring to FIG. 18B. As shown in FIG. 18B, the positionof the negative lens group 21 in the direction of the optical axis 201 ais shifted to Y1 due to the gravitational displacement of the negativelens group 21 and the lens holder 24. The position Y1 deviates from theposition Y0, which is not affected by the gravitation displacement ofthe negative lens group 21 and the lens holder 24, by a distance α inthe direction of the optical axis 201 a. Therefore, the space betweenthe positive and negative lens groups 22, 21 is A+α.

As described above, when spherical aberration is corrected by changingthe space between two lens groups, there is a problem that sphericalaberration is caused in the initial state due to a positional deviationα resulting from the gravitational displacement that depends on theorientation of the apparatus.

SUMMARY OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the presentinvention to provide an optical disk apparatus that can obtain afavorable focus error signal and thus perform stable focusing control,the apparatus including an objective lens with a high numerical aperture(preferably, 0.8 or more) to increase the recording density of anoptical disk and a device for correcting spherical aberration other thanthe objective lens used in focusing control, and correcting sphericalaberration for a recording plane to be subjected to focusing controlbefore operating the focusing control.

To achieve the above object, an optical disk apparatus of the presentinvention has the following configuration.

A first optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, atransfer system for transferring the focusing optical system in thedirection substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the transfersystem based on an output signal from the focusing error detectiondevice; a disk discrimination device for discriminating the type of theoptical disk; and an aberration correction quantity switching device forswitching a quantity of spherical aberration correction of theaberration correcting system selectively based on a signal from the diskdiscrimination device. The aberration correcting system includes aliquid crystal element. The quantity of spherical aberration correctionof the aberration correcting system is preset based on an output signalfrom the aberration correction quantity switching device beforeoperating the focusing control device.

In the first optical disk apparatus, it is preferable that the quantityof spherical aberration correction of the aberration correcting systemis determined based on a standard thickness of an intermediate layer ofa two-layer disk.

A second optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, atransfer system for transferring the focusing optical system in thedirection substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the transfersystem based on an output signal from the focusing error detectiondevice; and a reference value storage device for storing a quantity ofspherical aberration correction of the aberration correcting systemobtained when spherical aberration is optimized for an optical diskhaving a reference thickness. The aberration correcting system includesa liquid crystal element. The quantity of spherical aberrationcorrection of the aberration correcting system is preset based on anoutput signal from the reference value storage device before operatingthe focusing control device.

A third optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, atransfer system for transferring the focusing optical system in thedirection substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the transfersystem based on an output signal from the focusing error detectiondevice; a disk discrimination device for discriminating the type of theoptical disk; an aberration correction quantity switching device forswitching a first quantity of spherical aberration correction of theaberration correcting system selectively based on a signal from the diskdiscrimination device; a reference value storage device for storing asecond quantity of spherical aberration correction of the aberrationcorrecting system obtained when spherical aberration is optimized for anoptical disk having a reference thickness; and an adder for adding thefirst and second quantities of spherical aberration correction. Theaberration correcting system includes a liquid crystal element. Aquantity of spherical aberration correction of the aberration correctingsystem is preset based on an output signal from the adder beforeoperating the focusing control device.

A fourth optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, atransfer system for transferring the focusing optical system in thedirection substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; and a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the transfersystem based on an output signal from the focusing error detectiondevice. The aberration correcting system includes a liquid crystalelement. A learning operation for spherical aberration correctionquantity is performed before operating the focusing control device. Thelearning operation includes the steps of obtaining a first amplitude ofthe output signal from the focusing error detection device, storing thefirst amplitude, obtaining a second amplitude of the output signal fromthe focusing error detection device after changing a quantity ofspherical aberration correction of the aberration correcting system, andcomparing the first amplitude with the second amplitude.

A fifth optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, atransfer system for transferring the focusing optical system in thedirection substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; and a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the transfersystem based on an output signal from the focusing error detectiondevice. The aberration correcting system includes a liquid crystalelement. A learning operation for spherical aberration correctionquantity is performed before operating the focusing control device. Thelearning operation includes the steps of obtaining a first amplitude ofa reproduction signal, storing the first amplitude, obtaining a secondamplitude of the reproduction signal after changing a quantity ofspherical aberration correction of the aberration correcting system, andcomparing the first amplitude with the second amplitude.

In the fourth and fifth optical disk apparatuses, it is preferable thatthe learning operation is performed on every recording layer of theoptical disk at the time the optical disk is installed in the opticaldisk apparatus or the time the apparatus is turned on.

A sixth optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, afirst transfer system for transferring the focusing optical system inthe direction substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the firsttransfer system based on an output signal from the focusing errordetection device; a disk discrimination device for discriminating thetype of the optical disk; and an aberration correction quantityswitching device for switching a quantity of spherical aberrationcorrection of the aberration correcting system selectively based on asignal from the disk discrimination device. The aberration correctingsystem includes a first lens group and a second lens group that arearranged between the laser source and the focusing optical system, and asecond transfer system for changing the space between the first andsecond lens groups by shifting one of the first and second lens groupsin the optical axis direction. The quantity of spherical aberrationcorrection of the aberration correcting system is preset based on anoutput signal from the aberration correction quantity switching devicebefore operating the focusing control device.

In the sixth optical disk apparatus, it is preferable that the quantityof spherical aberration correction of the aberration correcting systemis determined based on a standard thickness of an intermediate layer ofa two-layer disk.

A seventh optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, afirst transfer system for transferring the focusing optical system inthe direction substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the firsttransfer system based on an output signal from the focusing errordetection device; and a reference value storage device for storing aquantity of spherical aberration correction of the aberration correctingsystem obtained when spherical aberration is optimized for an opticaldisk having a reference thickness. The aberration correcting systemincludes a first lens group and a second lens group that are arrangedbetween the laser source and the focusing optical system, and a secondtransfer system for changing the space between the first and second lensgroups by shifting one of the first and second lens groups in theoptical axis direction. The quantity of spherical aberration correctionof the aberration correcting system is preset based on an output signalfrom the reference value storage device before operating the focusingcontrol device.

An eighth optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, afirst transfer system for transferring the focusing optical system inthe direction substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the firsttransfer system based on an output signal from the focusing errordetection device; a disk discrimination device for discriminating thetype of the optical disk; an aberration correction quantity switchingdevice for switching a first quantity of spherical aberration correctionof the aberration correcting system selectively based on a signal fromthe disk discrimination device; a reference value storage device forstoring a second quantity of spherical aberration correction of theaberration correcting system obtained when spherical aberration isoptimized for an optical disk having a reference thickness; and an adderfor adding the first and second quantities of spherical aberrationcorrection. The aberration correcting system includes a first lens groupand a second lens group that are arranged between the laser source andthe focusing optical system, and a second transfer system for changingthe space between the first and second lens groups by shifting one ofthe first and second lens groups in the optical axis direction. Aquantity of spherical aberration correction of the aberration correctingsystem is preset based on an output signal from the adder beforeoperating the focusing control device.

In the seventh and eighth optical disk apparatuses, it is preferablethat the apparatuses further include a device for storing a quantity ofgravitational displacement correction used to correct the space betweenthe first and second lens groups.

A ninth optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, afirst transfer system for transferring the focusing optical system inthe direction substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; and a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the firsttransfer system based on an output signal from the focusing errordetection device. The aberration correcting system includes a first lensgroup and a second lens group that are arranged between the laser sourceand the focusing optical system, and a second transfer system forchanging the space between the first and second lens groups by shiftingone of the first and second lens groups in the optical axis direction. Alearning operation for spherical aberration correction quantity isperformed before operating the focusing control device. The learningoperation includes the steps of obtaining a first amplitude of theoutput signal from the focusing error detection device, storing thefirst amplitude, obtaining a second amplitude of the output signal fromthe focusing error detection device after changing a quantity ofspherical aberration correction of the aberration correcting system, andcomparing the first amplitude with the second amplitude.

A tenth optical disk apparatus of the present invention includes thefollowing: an optical pickup that includes a laser source, a focusingoptical system for receiving a light beam emitted from the laser sourceand focusing the light beam on an optical disk to form a tiny spot, afirst transfer system for transferring the focusing optical system inthe direction substantially perpendicular to the optical disk, aphotodetector for receiving light reflected from the optical disk andoutputting an electric signal in accordance with the quantity of light,and an aberration correcting system for correcting the sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; and a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the firsttransfer system based on an output signal from the focusing errordetection device. The aberration correcting system includes a first lensgroup and a second lens group that are arranged between the laser sourceand the focusing optical system, and a second transfer system forchanging the space between the first and second lens groups by shiftingone of the first and second lens groups in the optical axis direction. Alearning operation for spherical aberration correction quantity isperformed before operating the focusing control device. The learningoperation includes the steps of obtaining a first amplitude of areproduction signal, storing the first amplitude, obtaining a secondamplitude of the reproduction signal after changing a quantity ofspherical aberration correction of the aberration correcting system, andcomparing the first amplitude with the second amplitude.

In the ninth and tenth optical disk apparatuses, it is preferable thatthe learning operation is performed on every recording layer of theoptical disk at the time the optical disk is installed in the opticaldisk apparatus or the time the apparatus is turned on.

In the sixth to tenth optical disk apparatuses, it is preferable thatthe first transfer system and the aberration correcting system arelocated at different positions on the optical pickup.

According to the first to tenth optical disk apparatuses, a suitablecorrection of spherical aberration for the recording plane of an opticaldisk that is subjected to focusing control is performed before operatingthe focusing control device. Therefore, even when information isrecorded/reproduced on a high-density optical disk with an objectivelens having a large NA, a favorable focus error signal can be provided,resulting in stable operation of the focusing control.

There are some cases in which spherical aberration varies with eachoptical pickup due to adjustment errors of the focusing optical systemitself or the entire optical system. In the second, third, seventh andeighth optical disk apparatuses, the quantity of correction required tocorrect the spherical aberration inherent in each optical pickup isstored in the reference value storage device. Thus, the sphericalaberration is corrected before operating the focusing control devicewhile considering the spherical aberration inherent in each opticalpickup. Therefore, a favorable focus error signal can be provided,resulting in stable operation of focusing control: Consequently, theaccuracy regarding the components and assembly of optical pickups can berelaxed, which leads to an enhancement in mass-production of the opticalpickups and also to a reduction in cost.

In the fourth, fifth, ninth and tenth optical disk apparatuses, thelearning operation for spherical aberration correction quantity isperformed before operating the focusing control device. Therefore, evenwhen an optical disk has non-uniform thickness and when sphericalaberration varies with each optical pickup due to adjustment errors ofthe focusing optical system itself or the entire optical system, theoptimum quantity of spherical aberration correction can be obtained toreduce the spherical aberration. Thus, even if there are a thicknesserror of the optical disk and spherical aberration inherent in theoptical pickup, a favorable focus error signal always can be provided,resulting in stable operation of focusing control. Consequently, theaccuracy regarding the components and assembly of optical pickups can berelaxed, which leads to an enhancement in mass-production of the opticalpickups and also to a reduction in cost.

The use of the gravitational displacement correction quantity storagedevice makes it possible to correct a change in the space between thefirst lens group and the second lens group, even if the aberrationcorrecting lens group is located with its optical axis vertical. Thus,spherical aberration caused by the gravitational displacement of thefirst or the second lens group is corrected before operating thefocusing control device. Therefore, a favorable focus error signal canbe always provided, regardless of the orientation of the optical diskapparatus, resulting in stable operation of focusing control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 1 of the present invention.

FIG. 2A is a schematic cross-sectional view of a single-layer disk.

FIG. 2B is a schematic cross-sectional view of a two-layer disk.

FIG. 3 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 2 of the present invention.

FIG. 4 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 3 of the present invention.

FIG. 5 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 4 of the present invention.

FIG. 6 is a graph showing an example of calculation of sphericalaberration caused by a base material thickness error of an optical disk.

FIGS. 7A and 7B are graphs showing an example of calculation of a focuserror signal for an optical disk having a base material thickness errorof −20 μm: FIG. 7A shows the focus error signal before correctingspherical aberration; FIG. 7B shows the focus error signal aftercorrecting spherical aberration.

FIG. 8 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 5 of the present invention.

FIG. 9 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 6 of the present invention.

FIG. 10 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 7 of the present invention.

FIG. 11 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 8 of the present invention.

FIG. 12 is a schematic cross-sectional view of an optical disk apparatusaccording to Embodiment 9 of the present invention.

FIG. 13 is a schematic cross-sectional view of an optical pickup used inthe embodiments of the present invention and in a conventional example.

FIG. 14 is a schematic cross-sectional view of a conventional opticaldisk apparatus.

FIG. 15 is a schematic cross-sectional view of the main portions of anoptical pickup used in the embodiments of the present invention and in aconventional example.

FIG. 16 is a schematic perspective view of a conventional multi-layeroptical disk.

FIG. 17 is a schematic cross-sectional view of the main portions of aconventional optical pickup.

FIGS. 18A and 18B are schematic cross-sectional views of the mainportions of an optical pickup used in the embodiments of the presentinvention and in a conventional example, where an aberration correctinglens group is located with its optical axis horizontal in FIG. 18A andvertical in FIG. 18B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, specific embodiments of an optical disk apparatus of thepresent invention will be described in detail by referring to thedrawings.

Embodiment 1

FIG. 1 shows the configuration of an optical disk apparatus according toEmbodiment 1 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 11, an aberration correctingelement driving circuit 8, a control circuit 118, a disk discriminationdevice 12, and an aberration correction quantity switching device 14.The optical pickup 11 is the same as that shown in FIG. 13, which hasbeen explained in the conventional example. The aberration correctingelement driving circuit 8 drives a liquid crystal aberration correctingelement (an aberration correcting system) 4. The control circuit 118receives a signal from the optical pickup 11 and drives an objectivelens 5. The disk discrimination device 12 discriminates the type of anoptical disk. The aberration correction quantity switching device 14selects and switches the quantity of spherical aberration to becorrected by the liquid crystal aberration correcting element 4 based ona disk discrimination signal 13 output from the disk discriminationdevice 12.

Since the optical pickup 11 has the same configuration as that shown inFIG. 13, the identical elements to those in FIG. 13 are denoted by thesame reference numerals and the detailed description will be omitted.

The following is an explanation of the aberration correction quantityswitching device 14. For example, the base material thickness of areference disk is 100 μm. The aberration correction quantity switchingdevice 14 has three predetermined aberration correction quantities: (a)a quantity of spherical aberration correction of 0 mλ; (b) a quantity ofspherical aberration correction for an optical disk whose base materialthickness is 10 μm thinner than that of the reference disk; and (c) aquantity of spherical aberration correction for an optical disk whosebase material thickness is 10 μm thicker than that of the referencedisk. The aberration correction quantity switching device 14 selects andswitches an appropriate quantity for correcting spherical aberrationamong the three quantities in accordance with the disk discriminationsignal 13 from the disk discrimination device 12.

FIGS. 2A and 2B are cross-sectional views showing examples of theconfigurations of optical disks: FIG. 2A illustrates a first opticaldisk (single-layer disk) 71 with a single recording layer, and FIG. 2Billustrates a second optical disk (two-layer disk) 75 with two recordinglayers.

The first optical disk 71 in FIG. 2A includes a base material 72, arecording layer 73 and a protective layer 74 to form the back of theoptical disk, which are stacked in this order from the optical pickupside. The base material 72 is made of a transparent material such asresin or the like. The thickness from the surface of the first opticaldisk 71 on the optical pickup side to the recording layer is 0.1 mm.

The second optical disk 75 in FIG. 2B includes a base material 76, an L0layer (first recording layer) 77, an intermediate layer 78, an L1 layer(second recording layer) 79, and a protective layer 80 to form the backof the optical disk, which are stacked in this order from the opticalpickup side. The base material 76 and the intermediate layer 78 are madeof a transparent material such as resin or the like. The thicknessesfrom the surface of the second optical disk 75 on the optical pickupside to the L0 layer and to the L1 layer are 0.09 mm and 0.11 mm,respectively.

Next, the procedure for correcting spherical aberration of thisembodiment will be described. The spherical aberration correctingoperation can start when, e.g., an optical disk is installed in theoptical disk apparatus or the apparatus is turned on. First, the diskdiscrimination device 12 discriminates the type of the optical disk.When it turns out that the optical disk has a single recording layer(the first optical disk 71), the aberration correction quantityswitching device 14 selects the aberration correction quantity (a)according to the instructions of the disk discrimination signal 13.Thus, the liquid crystal aberration correcting element 4 corrects aspherical aberration that corresponds to a base material thickness errorof 0 μm relative to the reference disk. Consequently, a stable focuserror signal can be provided in focusing control that is performed onthe recording layer after the correction.

Next, the correction of spherical aberration for a two-layer disk willbe described.

When the optical disk having two recording layers (the second opticaldisk 75) is discriminated by the disk discrimination device 12 andfocusing control is performed on the L0 layer 77, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (b) according to the instructions of the diskdiscrimination signal 13. Thus, the liquid crystal aberration correctingelement 4 corrects a spherical aberration that corresponds to a basematerial thickness error of 10 μm by which the base material of theoptical disk is thinner than that of the reference disk. Consequently, astable focus error signal can be provided in focusing control that isperformed on the L0 layer 77 after the correction.

Similarly, when the optical disk having two recording layers (the secondoptical disk 75) is discriminated by the disk discrimination device 12and focusing control is performed on the L1 layer 79, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (c) according to the instructions of the diskdiscrimination signal 13. Thus, the liquid crystal aberration correctingelement 4 corrects a spherical aberration that corresponds to a basematerial thickness error of 10 μm by which the base material of theoptical disk is thicker than that of the reference disk. Consequently, astable focus error signal can be provided in focusing control that isperformed on the L1 layer 79 after the correction.

In an optical disk apparatus of this embodiment, the correction ofspherical aberration for the recording plane of an optical disk that issubjected to focusing control is performed before operating the focusingcontrol. Thus, a favorable focus error signal can be provided in thesubsequent focusing control, so that the effect of operating stablefocusing control can be achieved.

This embodiment starts the correction of spherical aberration beforeoperating focusing control. The quantity of spherical aberrationcorrection is predetermined for each recording plane of an optical diskthat is subjected to focusing control, among which an appropriatequantity is selected and switched in accordance with the type of thedisk and the target recording plane. In this embodiment, it is estimatedthat the quantity of correction to be selected by the aberrationcorrection quantity switching device 14 corresponds to a base materialthickness error of ±10 μm relative to the reference disk. However, thepresent invention is not limited to this base material thickness error.For example, the same result can be obtained even when the quantity ofaberration correction is determined based on a standard thickness of anintermediate layer of a two-layer disk. Specifically, it is possiblethat the quantity of spherical aberration correction for one recordingplane of the two-layer disk is set to 0 mλ, and that for the otherrecording plane is set while considering the standard thickness of theintermediate layer.

As the method for discriminating the type of an optical disk with thedisk discrimination device 12 in this embodiment, any method can be usedas long as it distinguishes the type of an optical disk, e.g., a methodin which discrimination is made by detecting a hole formed in acartridge for housing an optical disk, by the shape of the cartridge, orby using the quantity of light reflected from the optical disk todistinguish between a single-layer disk and a two-layer disk.

Embodiment 2

The following is an explanation of Embodiment 2. Here, the identicalelements to those in Embodiment 1 are denoted by the same referencenumerals and the detailed description will be omitted.

FIG. 3 shows the configuration of an optical disk apparatus according toEmbodiment 2 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 11, an aberration correctingelement driving circuit 8, a control circuit 118, and a reference valuestorage device 16. The optical pickup 11 is the same as that shown inFIG. 13, which has been explained in the conventional example. Theaberration correcting element driving circuit 8 drives a liquid crystalaberration correcting element (an aberration correcting system) 4. Thecontrol circuit 118 receives a signal from the optical pickup 11 anddrives an objective lens 5. The reference value storage device 16 storesthe quantity of spherical aberration correction obtained when the liquidcrystal aberration correcting element 4 corrects spherical aberrationoptimally for the optical disk having a reference thickness.

The reference value storage device 16 prestores the quantity ofspherical aberration corrected by the liquid crystal aberrationcorrecting element 4 when the optical pickup 11 is assembled andadjusted with the optical disk having a reference thickness (e.g., abase material thickness of 100 μm). The reference thickness of a diskmay be set by the standard value of a base material thickness of thefirst optical disk 71 in FIG. 2A or the second optical disk 75 in FIG.2B.

As the reference value storage device 16, any means can be used as longas it stores a second quantity of spherical aberration correction, e.g.,a variable resistor, a flash memory or EEPROM.

Next, the procedure for correcting spherical aberration of thisembodiment will be described. The spherical aberration correctingoperation can start when, e.g., an optical disk is installed in theoptical disk apparatus or the apparatus is turned on. An output signalfrom the reference value storage device 16 is input to the aberrationcorrecting element driving circuit 8. The liquid crystal aberrationcorrecting element 4 corrects the spherical aberration while consideringthe spherical aberration inherent in each of different optical pickups11. Therefore, a stable focus error signal can be provided in focusingcontrol that is performed on the recording layer after the correction.

As described above, in an optical disk apparatus of this embodiment, thereference value storage device 16 prestores the quantity of sphericalaberration correction of the liquid crystal aberration correctingelement 4 obtained when the optical pickup is assembled and adjustedwith the optical disk having a reference thickness. Using this quantityof spherical aberration correction, the correction of sphericalaberration for the recording plane of an optical disk that is subjectedto focusing control is performed before operating the focusing control.Thus, a favorable focus error signal can be provided in the subsequentfocusing control, so that the effect of operating stable focusingcontrol can be achieved.

There are some cases in which spherical aberration varies with eachoptical pickup due to adjustment errors during assembly of a lenselement and an optical pickup. In this embodiment, the quantity ofcorrection (which differs depending on the optical pickup) required tocorrect the spherical aberration inherent in the optical pickup has beenstored in the reference value storage device 16. Thus, the sphericalaberration is corrected before operating focusing control whileconsidering the spherical aberration inherent in the optical pickup.This makes it possible to provide a stable focus error signal amplitudein the subsequent focusing control.

Embodiment 3

The following is an explanation of Embodiment 3. Here, the identicalelements to those in Embodiments 1 and 2 are denoted by the samereference numerals and the detailed description will be omitted.

FIG. 4 shows the configuration of an optical disk apparatus according toEmbodiment 3 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 11, an aberration correctingelement driving circuit 8, a control circuit 118, a disk discriminationdevice 12, an aberration correction quantity switching device 14, areference value storage device 16, and an adder 17. The optical pickup11 is the same as that shown in FIG. 13, which has been explained in theconventional example. The aberration correcting element driving circuit8 drives a liquid crystal aberration correcting element (an aberrationcorrecting system) 4. The control circuit 118 receives a signal from theoptical pickup 11 and drives an objective lens 5. The diskdiscrimination device 12 discriminates the type of an optical disk. Theaberration correction quantity switching device 14 selects and switchesthe quantity of spherical aberration to be corrected by the liquidcrystal aberration correcting element 4 (a first quantity of sphericalaberration correction) based on a disk discrimination signal 13 outputfrom the disk discrimination device 12. The reference value storagedevice 16 stores the quantity of spherical aberration correction (asecond quantity of spherical aberration correction) obtained when theliquid crystal aberration correcting element 4 corrects sphericalaberration optimally for the optical disk having a reference thickness.The adder 17 is a circuit for adding the first and second quantities ofspherical aberration correction, the first quantity being an outputsignal from the aberration correction quantity switching device 14 andthe second quantity being an output signal from the reference valuestorage device 16.

The reference value storage device 16 prestores the quantity ofspherical aberration corrected by the liquid crystal aberrationcorrecting element 4 when the optical pickup 11 is assembled andadjusted with the optical disk having a reference thickness (e.g., abase material thickness of 100 μm) as the second quantity of sphericalaberration correction. The reference thickness of a disk may be set bythe standard value of a base material thickness of the first opticaldisk 71 in FIG. 2A or the second optical disk 75 in FIG. 2B.

As the reference value storage device 16, any means can be used as longas it stores the second quantity of spherical aberration correction,e.g., a variable resistor, a flash memory or EEPROM, and the same effectcan be obtained.

Next, the procedure for correcting spherical aberration of thisembodiment will be described. The spherical aberration correctingoperation can start when, e.g., an optical disk is installed in theoptical disk apparatus or the apparatus is turned on. First, the diskdiscrimination device 12 discriminates the type of the optical disk.When it turns out that the optical disk has a single recording layer(the first optical disk 71), the aberration correction quantityswitching device 14 selects the aberration correction quantity (a)according to the instructions of the disk discrimination signal 13. Thequantity (a) is used to correct a spherical aberration that correspondsto a base material thickness error of 0 μm relative to the referencedisk. Then, the output signal (i.e., the first quantity of sphericalaberration correction) from the aberration correction quantity switchingdevice 14 is input to the adder 17. Moreover, the output signal (i.e.,the second quantity of spherical aberration correction) from thereference value storage device 16 also is input to the adder 17. Theadder 17 adds the first and second quantities and outputs the result tothe aberration correcting element driving circuit 8. The liquid crystalaberration correcting element 4 corrects spherical aberration whileconsidering the spherical aberration inherent in each of differentoptical pickups 11. Therefore, a stable focus error signal can beprovided in focusing control that is performed on the recording layerafter the correction.

Next, the correction of spherical aberration for a two-layer disk willbe described.

When the optical disk having two recording layers (the second opticaldisk 75) is discriminated by the disk discrimination device 12 andfocusing control is performed on the L0 layer 77, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (b) according to the instructions of the diskdiscrimination signal 13. The quantity (b) is used to correct aspherical aberration that corresponds to a base material thickness errorof 10 μm by which the base material of the optical disk is thinner thanthat of the reference disk. Then, the output signal (i.e., the firstquantity of spherical aberration correction) from the aberrationcorrection quantity switching device 14 is input to the adder 17.Moreover, the output signal (i.e., the second quantity of sphericalaberration correction) from the reference value storage device 16 alsois input to the adder 17. The adder 17 adds the first and secondquantities and outputs the result to the aberration correcting elementdriving circuit 8. The liquid crystal aberration correcting element 4corrects spherical aberration while considering the spherical aberrationinherent in each of different optical pickups 11. Therefore, a stablefocus error signal can be provided in focusing control that is performedon the L0 layer 77 after the correction.

Similarly, when the optical disk having two recording layers (the secondoptical disk 75) is discriminated by the disk discrimination device 12and focusing control is performed on the L1 layer 79, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (c) according to the instructions of the diskdiscrimination signal 13. The quantity (c) is used to correct aspherical aberration that corresponds to a base material thickness errorof 10 μm by which the base material of the optical disk is thicker thanthat of the reference disk. Then, the output signal (i.e., the firstquantity of spherical aberration correction) from the aberrationcorrection quantity switching device 14 is input to the adder 17.Moreover, the output signal (i.e., the second quantity of sphericalaberration correction) from the reference value storage device 16 alsois input to the adder 17. The adder 17 adds the first and secondquantities and outputs the result to the aberration correcting elementdriving circuit 8. The liquid crystal aberration correcting element 4corrects spherical aberration while considering the spherical aberrationinherent in each of different optical pickups 11. Therefore, a stablefocus error signal can be provided in focusing control that is performedon the L1 layer 79 after the correction.

As described above, in an optical disk apparatus of this embodiment, thereference value storage device 16 prestores the quantity of sphericalaberration correction (the second quantity) of the liquid crystalaberration correcting element 4 obtained when the optical pickup isassembled and adjusted with the optical disk having a referencethickness. Further, the quantity of spherical aberration correction (thefirst quantity) that corresponds to the type of the disk and therecording plane to be subjected to focusing control is selected. Usingthe quantity of spherical aberration correction obtained by adding thefirst and second quantities, the correction of spherical aberration forthe recording plane of an optical disk that is subjected to focusingcontrol is performed before operating the focusing control. Thus, afavorable focus error signal can be provided in the subsequent focusingcontrol, so that the effect of operating stable focusing control can beachieved.

There are some cases in which spherical aberration varies with eachoptical pickup due to adjustment errors during assembly of a lenselement and an optical pickup. In this embodiment, the quantity ofcorrection (which differs depending on the optical pickup) required tocorrect the spherical aberration inherent in the optical pickup has beenstored in the reference value storage device 16. Thus, sphericalaberration is corrected before operating focusing control whileconsidering the spherical aberration inherent in the optical pickup.This makes it possible to provide a stable focus error signal amplitudein the subsequent focusing control.

This embodiment starts the correction of spherical aberration beforeoperating focusing control. The quantity of spherical aberrationcorrection is predetermined for each recording plane of an optical diskthat is subjected to focusing control, among which an appropriatequantity is selected and switched in accordance with the type of theoptical disk and the target recording plane.

In the above example, the reference value storage device 16 stores thequantity of spherical aberration correction of the liquid crystalaberration correcting element 4 obtained when the optical pickup isassembled and adjusted with one optical disk. However, the presentinvention is not limited thereto. For example, the reference valuestorage device 16 may store a plurality of quantities of sphericalaberration correction (the second quantity of spherical aberrationcorrection) that are optimized for each type of reference optical disks,such as a two-layer disk and a single-layer disk with different basematerial thicknesses, and the second quantity of spherical aberrationcorrection corresponding to the result of discrimination by the diskdiscrimination device 12 can be input to the adder 17.

As the method for discriminating the type of an optical disk with thedisk discrimination device 12 in this embodiment, any method can be usedas long as it distinguishes the type of an optical disk, e.g., a methodin which discrimination is made by detecting a hole formed in acartridge for housing an optical disk, by the shape of the cartridge, orby using the quantity of light reflected from the optical disk todistinguish between a single-layer disk and a two-layer disk.

Embodiment 4

The following is an explanation of Embodiment 4. Here, the identicalelements to those in Embodiments 1 to 3 are denoted by the samereference numerals and the detailed description will be omitted.

FIG. 5 shows the configuration of an optical disk apparatus according toEmbodiment 4 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 11, an aberration correctingelement driving circuit 8, a control circuit 118, a disk discriminationdevice 12, an FE signal generation circuit 31, an FE amplitude detector32, a storage device 34, and an FE amplitude comparator 33. The opticalpickup 11 is the same as that shown in FIG. 13, which has been explainedin the conventional example. The aberration correcting element drivingcircuit 8 drives a liquid crystal aberration correcting element (anaberration correcting system) 4. The control circuit 118 receives asignal from the optical pickup 11 and drives an objective lens 5. Thedisk discrimination device 12 discriminates the type of an optical disk.The FE signal generation circuit 31 generates a focus error (FE) signal.The FE amplitude detector 32 detects the amplitude of the focus errorsignal. The storage device 34 stores the focus error signal. The FEamplitude comparator 33 makes a comparison between amplitudes of thefocus error signals before and after changing the quantity of sphericalaberration correction of the liquid crystal aberration correctingelement 4.

FIG. 6 shows an example of the calculation of spherical aberrationcaused by a base material thickness error of an optical disk 6. Thedotted line indicates the spherical aberration before correction, andthe solid line indicates the spherical aberration after correction. Abase material thickness error of ±20 μm causes a spherical aberration ofabout 190 mλ, and the correction of spherical aberration allows thespherical aberration to be corrected properly as represented by thesolid line in FIG. 6.

FIGS. 7A and 7B show examples of the calculation of a focus error signalfor the optical disk having a base material thickness error of −20 μm.The focus error signals plotted on the vertical axis are standardized bythe amplitude of a focus error signal when spherical aberration iscorrected to 0 mλ. The horizontal axis represents the distance between arecording layer of the optical disk and the objective lens. Theobjective lens used for the calculation has a numerical aperture of0.85.

FIG. 7A shows a focus error signal before correcting sphericalaberration. The base material thickness error of −20 {dot over (μ)}mcauses a spherical aberration of about 190 mλ, as shown in FIG. 6.Therefore, the amplitude of the focus error signal is reduced and theshape becomes asymmetrical due to the influence of this sphericalaberration.

FIG. 7B shows a focus error signal after correcting the sphericalaberration, in which a considerable improvement in amplitude and shapeof the focus error signal is achieved.

As can be seen from FIGS. 7A and 7B, the correction of sphericalaberration increases the amplitude of a focus error signal and improvesthe linearity and symmetry in the shape of the focus error signal, thusproviding a favorable focus error signal. In this embodiment, thequantity of correction required to correct spherical aberration causedby base material. thickness errors that differ depending on the opticaldisk is predetermined, and spherical aberration is corrected with thisquantity, followed by focusing control.

Next, the procedure for correcting spherical aberration of thisembodiment will be described.

First, the disk discrimination device 12 discriminates the type of theoptical disk. For example, it determines whether the optical diskinstalled in the optical disk apparatus is a single-layer disk (FIG. 2A)or a two-layer disk (FIG. 2B). In the case of a multi-layer disk, itdetermines which recording layer is subjected to focusing control.

The correction of spherical aberration for the optical disk 71 in FIG.2A will be described as an example. First, the amplitude of a focuserror signal output from the FE signal generation circuit 31 (a firstamplitude FE0) is measured by the FE amplitude detector 32 and stored inthe storage device 34. Then, the quantity of spherical aberrationcorrection of the liquid crystal aberration correcting element 4 ischanged. The amplitude of a focus error signal output from the FE signalgeneration circuit 31 after the change (a second amplitude FE1) ismeasured by the FE amplitude detector 32. The FE amplitude comparator 33compares the first amplitude FE0 stored in the storage device 34 withthe second amplitude FE1. Thereafter, such a series of steps is repeateduntil the amplitude of the focus error signal reaches to a maximum. Thequantity of spherical aberration correction of the liquid crystalaberration correcting element 4 obtained when the amplitude of the focuserror signal is maximized is optimum for the recording layer of thefirst optical disk 71.

In an optical disk apparatus of the present invention, a learningoperation for acquiring the optimum quantity of spherical aberrationcorrection for the recording plane of an optical disk that is subjectedto focusing control is performed before operating the focusing control.Then, the liquid crystal aberration correcting element 4 correctsaberration with the optimum quantity of spherical aberration correctionthus obtained, followed by focusing control. Consequently, a favorablefocus error signal can be provided in focusing control, so that theeffect of operating stable focusing control can be achieved.

The optical disk apparatus of this embodiment starts the correction ofspherical aberration before operating focusing control. The learningoperation for acquiring the optimum quantity of spherical aberrationcorrection can be performed each time immediately before the focusingcontrol starts. Alternately, e.g., the learning operation for acquiringthe optimum quantities of spherical aberration correction for everyrecording layer can be performed when an optical disk is installed inthe optical disk apparatus or the apparatus is turned on, and theobtained quantities that correspond to the respective recording layersmay be stored in a memory.

The above example has described the method in which the amplitude of afocus error signal is maximized to acquire the optimum quantity ofspherical aberration correction of the liquid crystal aberrationcorrecting element 4. However, the present invention is not limitedthereto. The same effect can be obtained, e.g., by a method formaximizing the amplitude of a reproduction signal instead of the focuserror signal, a method for maximizing the entire quantity of light, orthe like.

As the method for discriminating the type of an optical disk with thedisk discrimination device 12 in this embodiment, any method can be usedas long as it distinguishes the type of an optical disk, e.g., a methodin which discrimination is made by detecting a hole formed in acartridge for housing an optical disk, by the shape of the cartridge, orby using the quantity of light reflected from the optical disk todistinguish between a single-layer disk and a two-layer disk.

Embodiment 5

FIG. 8 shows the configuration of an optical disk apparatus according toEmbodiment 5 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 51, an aberration correctingelement driving circuit 8, a control circuit 118, a disk discriminationdevice 12, and an aberration correction quantity switching device 14.The optical pickup 51 uses an aberration correcting lens group (anaberration correcting system) 201 instead of the liquid crystalaberration correcting element 4 of the optical pickup 11 shown in FIG.13, which has been explained in the conventional example. The aberrationcorrecting element driving circuit 8 drives the aberration correctinglens group 201. The control circuit 118 receives a signal from theoptical pickup 51 and drives an objective lens 5. The diskdiscrimination device 12 discriminates the type of an optical disk. Theaberration correction quantity switching device 14 selects and switchesthe quantity of spherical aberration to be corrected by the aberrationcorrecting lens group 201 based on a disk discrimination signal 13output from the disk discrimination device 12.

The aberration correcting lens group 201 includes two lens groups,composed of a positive lens group 22 and a negative lens group 21, and adriving portion 25 for shifting the negative lens group 21 in theoptical axis direction.

Since the optical pickup 51 has the same configuration as that shown inFIG. 13 except for the aberration correcting lens group 201, theidentical elements to those in FIG. 13 are denoted by the same referencenumerals and the detailed description will be omitted.

The following is an explanation of the aberration correction quantityswitching device 14. For example, the base material thickness of areference disk is 100 μm. The aberration correction quantity switchingdevice 14 has three predetermined aberration correction quantities: (a)a quantity of spherical aberration correction of 0 mλ; (b) a quantity ofspherical aberration correction for an optical disk whose base materialthickness is 10 μm thinner than that of the reference disk; and (c) aquantity of spherical aberration correction for an optical disk whosebase material thickness is 10 μm thicker than that of the referencedisk. The aberration correction quantity switching device 14 selects andswitches an appropriate quantity for correcting spherical aberrationamong the three quantities in accordance with the disk discriminationsignal 13 from the disk discrimination device 12.

Next, the procedure for correcting spherical aberration of thisembodiment will be described. The spherical- aberration correctingoperation can start when, e.g., an optical disk is installed in theoptical disk apparatus or the apparatus is turned on. First, the diskdiscrimination device 12 discriminates the type of the optical disk.When it turns out that the optical disk has a single recording layer(the first optical disk 71), the aberration correction quantityswitching device 14 selects the aberration correction quantity (a)according to the instructions of the disk discrimination signal 13.Thus, the aberration correcting lens group 201 corrects a sphericalaberration that corresponds to a base material thickness error of 0 μmrelative to the reference disk. Consequently, a stable focus errorsignal can be provided in focusing control that is performed on therecording layer after the correction.

Next, the correction of spherical aberration for a two-layer disk willbe described.

When the optical disk having two recording layers (the second opticaldisk 75) is discriminated by the disk discrimination device 12 andfocusing control is performed on the L0 layer 77, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (b) according to the instructions of the diskdiscrimination signal 13. Thus, the aberration correcting lens group 201corrects a spherical aberration that corresponds to a base materialthickness error of 10 μm by which the base material of the optical diskis thinner than that of the reference disk. Consequently, a stable focuserror signal can be provided in focusing control that is performed onthe L0 layer 77 after the correction.

Similarly, when the optical disk having two recording layers (the secondoptical disk 75) is discriminated by the disk discrimination device 12and focusing control is performed on the L1 layer 79, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (c) according to the instructions of the diskdiscrimination signal 13. Thus, the aberration correcting lens group 201corrects a spherical aberration that corresponds to a base materialthickness error of 10 μm by which the base material of the optical diskis thicker than that of the reference disk. Consequently, a stable focuserror signal can be provided in focusing control that is performed onthe L1 layer 79 after the correction.

In an optical disk apparatus of this embodiment, the correction ofspherical aberration for the recording plane of an optical disk that issubjected to focusing control is performed before operating the focusingcontrol. Thus, a favorable focus error signal can be provided in thesubsequent focusing control, so that the effect of operating stablefocusing control can be achieved.

This embodiment starts the correction of spherical aberration beforeoperating focusing control. The quantity of spherical aberrationcorrection is predetermined for each recording plane of an optical diskthat is subjected to focusing control, among which an appropriatequantity is selected and switched in accordance with the type of thedisk and the target recording plane. In this embodiment, it is estimatedthat the quantity of correction to be selected by the aberrationcorrection quantity switching device 14 corresponds to a base materialthickness error of ±10 μm relative to the reference disk. However, thepresent invention is not limited to this base material thickness error.For example, the same result can be obtained even when the quantity ofaberration correction is determined based on a standard thickness of anintermediate layer of a two-layer disk. Specifically, it is possiblethat the quantity of spherical aberration correction for one recordingplane of the two-layer disk is set to 0 mλ, and that for the otherrecording plane is set while considering the standard thickness of theintermediate layer.

As the method for discriminating the type of an optical disk with thedisk discrimination device 12 in this embodiment, any method can be usedas long as it distinguishes the type of an optical disk, e.g., a methodin which discrimination is made by detecting a hole formed in acartridge for housing an optical disk, by the shape of the cartridge, orby using the quantity of light reflected from the optical disk todistinguish between a single-layer disk and a two-layer disk.

Embodiment 6

The following is an explanation of Embodiment 6. Here, the identicalelements to those in Embodiments 1 to 5 are denoted by the samereference numerals and the detailed description will be omitted.

FIG. 9 shows the configuration of an optical disk apparatus according toEmbodiment 6 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 51, an aberration correctingelement driving circuit 8, a control circuit 118, and a reference valuestorage device 16. The optical pickup 51 is the same as that inEmbodiment 5. The aberration correcting element driving circuit 8 drivesan aberration correcting lens group (an aberration correcting system)201. The control circuit 118 receives a signal from the optical pickup51 and drives an objective lens 5. The reference value storage device 16stores the quantity of spherical aberration correction obtained when theaberration correcting lens group 201 corrects spherical aberrationoptimally for the optical disk having a reference thickness.

The aberration correcting lens group 201 includes two lens groups,composed of a positive lens group 22 and a negative lens group 21, and adriving portion 25 for shifting the negative lens group 21 in theoptical axis direction.

The reference value storage device 16 prestores the quantity ofspherical aberration corrected by the aberration correcting lens group201 when the optical pickup 51 is assembled and adjusted with theoptical disk having a reference thickness (e.g., a base materialthickness of 100 μm). The reference thickness of a disk may be set bythe standard value of a base material thickness of the first opticaldisk 71 in FIG. 2A or the second optical disk 75 in FIG. 2B.

As the reference value storage device 16, any means can be used as longas it stores a second quantity of spherical aberration correction, e.g.,a variable resistor, a flash memory or EEPROM.

Next, the procedure for correcting spherical aberration of thisembodiment will be described. The spherical aberration correctingoperation can start when, e.g., an optical disk is installed in theoptical disk apparatus or the apparatus is turned on. An output signalfrom the reference value storage device 16 is input to the aberrationcorrecting element driving circuit 8. The aberration correcting lensgroup 201 corrects spherical aberration while considering the sphericalaberration inherent in each of different optical pickups 11. Therefore,a stable focus error signal can be provided in focusing control that isperformed on the recording layer after the correction.

As described above, in an optical disk apparatus of this embodiment, thereference value storage device 16 prestores the quantity of sphericalaberration correction of the aberration correcting lens group 201obtained when the optical pickup is assembled and adjusted with theoptical disk having a reference thickness. Using this quantity ofspherical aberration correction, the correction of spherical aberrationfor the recording plane of an optical disk that is subjected to focusingcontrol is performed before operating the focusing control. Thus, afavorable focus error signal can be provided in the subsequent focusingcontrol, so that the effect of operating stable focusing control can beachieved.

There are some cases in which spherical aberration varies with eachoptical pickup due to adjustment errors during assembly of a lenselement and an optical pickup. In this embodiment, the quantity ofcorrection (which differs depending on the optical pickup) required tocorrect the spherical aberration inherent in the optical pickup has beenstored in the reference value storage device 16. Thus, sphericalaberration is corrected before operating focusing control whileconsidering the spherical aberration inherent in the optical pickup.This makes it possible to provide a stable focus error signal amplitudein the subsequent focusing control.

Embodiment 7

The following is an explanation of Embodiment 7. Here, the identicalelements to those in Embodiments 1 to 6 are denoted by the samereference numerals and the detailed description will be omitted.

FIG. 10 shows the configuration of an optical disk apparatus accordingto Embodiment 7 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 51, an aberration correctingelement driving circuit 8, a control circuit 118, a disk discriminationdevice 12, an aberration correction quantity switching device 14, areference value storage device 16, and an adder 17. The optical pickup51 is the same as that in Embodiment 5. The aberration correctingelement driving circuit 8 drives an aberration correcting lens group (anaberration correcting system) 201. The control circuit 118 receives asignal from the optical pickup 51 and drives an objective lens 5. Thedisk discrimination device 12 discriminates the type of an optical disk.The aberration correction quantity switching device 14 selects andswitches the quantity of spherical aberration to be corrected by theaberration correcting lens group 201 (a first quantity of sphericalaberration correction) based on a disk discrimination signal 13 outputfrom the disk discrimination device 12. The reference value storagedevice 16 stores the quantity of spherical aberration correction (asecond quantity of spherical aberration correction) obtained when theaberration correcting lens group 201 corrects spherical aberrationoptimally for the optical disk having a reference thickness. The adder17 is a circuit for adding the first and second quantities of sphericalaberration correction, the first quantity being an output signal fromthe aberration correction quantity switching device 14 and the secondquantity being an output signal from the reference value storage device16.

The aberration correcting lens group 201 includes two lens groups,composed of a positive lens group 22 and a negative lens group 21, and adriving portion 25 for shifting the negative lens group 21 in theoptical axis direction.

The reference value storage device 16 prestores the quantity ofspherical aberration corrected by the aberration correcting lens group201 when the optical pickup 51 is assembled and adjusted with theoptical disk having a reference thickness (e.g., a base materialthickness of 100 μm) as the second quantity of spherical aberrationcorrection. The reference thickness of a disk may be set by the standardvalue of a base material thickness of the first optical disk 71 in FIG.2A or the second optical disk 75 in FIG. 2B.

As the reference value storage device 16, any means can be used as longas it stores the second quantity of spherical aberration correction,e.g., a variable resistor, a flash memory or EEPROM, and the same effectcan be obtained.

Next, the procedure for correcting spherical aberration of thisembodiment will be described. The spherical aberration correctingoperation can start when, e.g., an optical disk is installed in theoptical disk apparatus or the apparatus is turned on. First, the diskdiscrimination device 12 discriminates the type of the optical disk.When it turns out that the optical disk has a single recording layer(the first optical disk 71), the aberration correction quantityswitching device 14 selects the aberration correction quantity (a)according to the instructions of the disk discrimination signal 13. Thequantity (a) is used to correct a spherical aberration that correspondsto a base material thickness error of 0 μm relative to the referencedisk. Then, the output signal (i.e., the first quantity of sphericalaberration correction) from the aberration correction quantity switchingdevice 14 is input to the adder 17. Moreover, the output signal (i.e.,the second quantity of spherical aberration correction) from thereference value storage device 16 also is input to the adder 17. Theadder 17 adds the first and second quantities and outputs the result tothe aberration correcting element driving circuit 8. The aberrationcorrecting lens group 201 corrects spherical aberration whileconsidering the spherical aberration inherent in each of differentoptical pickups 11. Therefore, a stable focus error signal can beprovided in focusing control that is performed on the recording layerafter the correction.

Next, the correction of spherical aberration for a two-layer disk willbe described.

When the optical disk having two recording layers (the second opticaldisk 75) is discriminated by the disk discrimination device 12 andfocusing control is performed on the L0 layer 77, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (b) according to the instructions of the diskdiscrimination signal 13. The quantity (b) is used to correct aspherical aberration that corresponds to a base material thickness errorof 10 μm by which the base material of the optical disk is thinner thanthat of the reference disk. Then, the output signal (i.e., the firstquantity of spherical aberration correction) from the aberrationcorrection quantity switching device 14 is input to the adder 17.Moreover, the output signal (i.e., the second quantity of sphericalaberration correction) from the reference value storage device 16 alsois input to the adder 17. The adder 17 adds the first and secondquantities and outputs the result to the aberration correcting elementdriving circuit 8. The aberration correcting lens group 201 correctsspherical aberration while considering the spherical aberration inherentin each of different optical pickups 11. Therefore, a stable focus errorsignal can be provided in focusing control that is performed on the L0layer 77 after the correction.

Similarly, when the optical disk having two recording layers (the secondoptical disk 75) is discriminated by the disk discrimination device 12and focusing control is performed on the L1 layer 79, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (c) according to the instructions of the diskdiscrimination signal 13. The quantity (c) is used to correct aspherical aberration that corresponds to a base material thickness errorof 10 μm by which the base material of the optical disk is thicker thanthat of the reference disk. Then, the output signal (i.e., the firstquantity of spherical aberration correction) from the aberrationcorrection quantity switching device 14 is input to the adder 17.Moreover, the output signal (i.e., the second quantity of sphericalaberration correction) from the reference value storage device 16 alsois input to the adder 17. The adder 17 adds the first and secondquantities and outputs the result to the aberration correcting elementdriving circuit 8. The aberration correcting lens group 201 correctsspherical aberration while considering the spherical aberration inherentin each of different optical pickups 11. Therefore, a stable focus errorsignal can be provided in focusing control that is performed on the L1layer 79 after the correction.

As described above, in an optical disk apparatus of this embodiment, thereference value storage device 16 prestores the quantity of sphericalaberration correction (the second quantity) of the aberration correctinglens group 201 obtained when the optical pickup is assembled andadjusted with the optical disk having a reference thickness. Further,the quantity of spherical aberration correction (the first quantity)that corresponds to the type of the disk and the recording plane to besubjected to focusing control is selected. Using the quantity ofspherical aberration correction obtained by adding the first and secondquantities, the correction of spherical aberration for the recordingplane of an optical disk that is subjected to focusing control isperformed before operating the focusing control. Thus, a favorable focuserror signal can be provided in the subsequent focusing control, so thatthe effect of operating stable focusing control can be achieved.

There are some cases in which spherical aberration varies with eachoptical pickup due to adjustment errors during assembly of a lenselement and an optical pickup. In this embodiment, the quantity ofcorrection (which differs depending on the optical pickup) required tocorrect the spherical aberration inherent in the optical pickup has beenstored in the reference value storage device 16. Thus, sphericalaberration is corrected before operating focusing control whileconsidering the spherical aberration inherent in the optical pickup.This makes it possible to provide a stable focus error signal amplitudein the subsequent focusing control.

This embodiment starts the correction of spherical aberration beforeoperating focusing control. The quantity of spherical aberrationcorrection is predetermined for each recording plane of an optical diskthat is subjected to focusing control, among which an appropriatequantity is selected and switched in accordance with the type of theoptical disk and the target recording plane.

In the above example, the reference value storage device 16 stores thequantity of spherical aberration correction of the aberration correctinglens group 201 obtained when the optical pickup is assembled andadjusted with one optical disk. However, the present invention is notlimited thereto. For example, the reference value storage device 16 maystore a plurality of quantities of spherical aberration correction (thesecond quantity of spherical aberration correction) that are optimizedfor each type of reference optical disks, such as a two-layer disk and asingle-layer disk with different base material thicknesses, and thesecond quantity of spherical aberration correction corresponding to theresult of discrimination by the disk discrimination device 12 can beinput to the adder 17.

As the method for discriminating the type of an optical disk with thedisk discrimination device 12 in this embodiment, any method can be usedas long as it distinguishes the type of an optical disk, e.g., a methodin which discrimination is made by detecting a hole formed in acartridge for housing an optical disk, by the shape of the cartridge, orby using the quantity of light reflected from the optical disk todistinguish between a single-layer disk and a two-layer disk.

Embodiment 8

The following is an explanation of Embodiment 8. Here, the identicalelements to those in Embodiments 1 to 7 are denoted by the samereference numerals and the detailed description will be omitted.

FIG. 11 shows the configuration of an optical disk apparatus accordingto Embodiment 8 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 51, an aberration correctingelement driving circuit 8, a control circuit 118, a disk discriminationdevice 12, an FE signal generation circuit 31, an FE amplitude detector32, a storage device 34, and an FE amplitude comparator 33. The opticalpickup 51 is the same as that in Embodiment 5. The aberration correctingelement driving circuit 8 drives an aberration correcting lens group (anaberration correcting system) 201. The control circuit receives a signalfrom the optical pickup 51 and drives an objective lens 5. The diskdiscrimination device 12 discriminates the type of an optical disk. TheFE signal generation circuit 31 generates a focus error (FE) signal. TheFE amplitude detector 32 detects the amplitude of the focus errorsignal. The storage device 34 stores the focus error signal. The FEamplitude comparator 33 makes a comparison between amplitudes of thefocus error signals before and after changing the quantity of sphericalaberration correction of the aberration correcting lens group 201.

The aberration correcting lens group 201 includes two lens groups,composed of a positive lens group 22 and a negative lens group 21, and adriving portion 25 for shifting the negative lens group 21 in theoptical axis direction.

Next, the procedure for correcting spherical aberration of thisembodiment will be described.

First, the disk discrimination device 12 discriminates the type of theoptical disk. For example, it determines whether the optical diskinstalled in the optical disk apparatus is a single-layer disk (FIG. 2A)or a two-layer disk (FIG. 2B). In the case of a multi-layer disk, itdetermines which recording layer is subjected to focusing control.

The correction of spherical aberration for the optical disk 71 in FIG.2A will be described as an example. First, the amplitude of a focuserror signal output from the FE signal generation circuit 31 (a firstamplitude FE0) is measured by the FE amplitude detector 32 and stored inthe storage device 34. Then, the quantity of spherical aberrationcorrection of the aberration correcting lens group 201 is changed. Theamplitude of a focus error signal output from the FE signal generationcircuit 31 after the change (a second amplitude FE1) is measured by theFE amplitude detector 32. The FE amplitude comparator 33 compares thefirst amplitude FE0 stored in the storage device 34 with the secondamplitude FE1. Thereafter, this series of steps is repeated until theamplitude of the focus error signal reaches a maximum. The quantity ofspherical aberration correction of the aberration correcting lens group201 obtained when the amplitude of the focus error signal is maximizedis optimum for the recording layer of the first optical disk 71.

In an optical disk apparatus of the present invention, a learningoperation for acquiring the optimum quantity of spherical aberrationcorrection for the recording plane of an optical disk that is subjectedto focusing control is performed before operating the focusing control.Then, the aberration correcting lens group 201 corrects aberration withthe optimum quantity of spherical aberration correction thus obtained,followed by focusing control.. Consequently, a favorable focus errorsignal can be provided in focusing control, so that the effect ofoperating stable focusing control can be achieved.

The optical disk apparatus of this embodiment starts the correction ofspherical aberration before operating focusing control. The learningoperation for acquiring the optimum quantity of spherical aberrationcorrection can be performed each time immediately before the focusingcontrol starts. Alternately, e.g., the learning operation for acquiringthe optimum quantities of spherical aberration correction for everyrecording layer can be performed when an optical disk is installed inthe optical disk apparatus or the apparatus is turned on, and theobtained quantities that correspond to the respective recording layersmay be stored in a memory.

The above example has described the method in which the amplitude of afocus error signal is maximized to acquire the optimum quantity ofspherical aberration correction of the aberration correcting lens group201. However, the present invention is not limited thereto. The sameeffect can be obtained, e.g., by a method for maximizing the amplitudeof a reproduction signal instead of the focus error signal, a method formaximizing the entire quantity of light, or the like.

As the method for discriminating the type of an optical disk with thedisk discrimination device 12 in this embodiment, any method can be usedas long as it distinguishes the type of an optical disk, e.g., a methodin which discrimination is made by detecting a hole formed in acartridge for housing an optical disk, by the shape of the cartridge, orby using the quantity of light reflected from the optical disk todistinguish between a single-layer disk and a two-layer disk.

Embodiment 9

The following is an explanation of Embodiment 9. Here, the identicalelements to those in Embodiments 1 to 8 are denoted by the samereference numerals and the detailed description will be omitted.

FIG. 12 shows the configuration of an optical disk apparatus accordingto Embodiment 9 of the present invention. The optical disk apparatus ofthis embodiment includes an optical pickup 51, an aberration correctingelement driving circuit 8, a control circuit 118, a disk discriminationdevice 12, an aberration correction quantity switching device 14, agravitational displacement correction quantity storage device 18, and anadder 17. The optical pickup 51 is the same as that in Embodiment 5. Theaberration correcting element driving circuit 8 drives an aberrationcorrecting lens group (an aberration correcting system) 201. The controlcircuit 118 receives a signal from the optical pickup 51 and drives anobjective lens 5. The disk discrimination device 12 discriminates thetype of an optical disk. The aberration correction quantity switchingdevice 14 selects and switches the quantity of spherical aberration tobe corrected by the aberration correcting lens group 201 (a firstquantity of spherical aberration correction) based on a diskdiscrimination signal 13 output from the disk discrimination device 12.The gravitational displacement correction quantity storage device 18stores the quantity of correction required to correct a change in thespace between a positive lens group 22 and a negative lens group 21,which constitute the aberration correcting lens group 201, due togravity (i.e., the quantity of gravitational displacement correction,namely, a third quantity of spherical aberration correction). The adder17 is a circuit for adding the first and third quantities of sphericalaberration correction, the first quantity being an output signal fromthe aberration correction quantity switching device 14 and the thirdquantity being an output signal from the gravitational displacementcorrection quantity storage device 18.

The aberration correcting lens group 201 includes two lens groups,composed of the positive lens group 22 and the negative lens group 21,and a driving portion 25 for shifting the negative lens group 21 in theoptical axis direction.

The following is an explanation of a configuration in which the opticalaxis of the aberration correcting lens group 201 is arranged in thevertical direction.

The gravitational displacement correction quantity storage device willbe described. As shown in FIG. 18B illustrating a conventionaltechnique, when the aberration correcting lens group including two lensgroups is located with its optical axis vertical, the position Y1 of thenegative lens group 21 deviates from the designed position Y0 by adistance a (i.e., positional deviation) because of the gravitationaldisplacement of the negative lens group 21 and the lens holder 24.Consequently, spherical aberration is caused by the positional deviationa. In this embodiment, the quantity of correction (the quantity ofgravitational displacement correction) required to correct the sphericalaberration due to the positional deviation α has been measured, whichthen is stored in the gravitational displacement correction quantitystorage device 18. Therefore, spherical aberration can be corrected bydriving the lens holder 24 so that the positional deviation a resultingfrom the gravitational displacement of the negative lens group 21 iscorrected.

Next, the procedure for correcting spherical aberration of thisembodiment will be described. The spherical aberration correctingoperation can start when, e.g., an optical disk is installed in theoptical disk apparatus or the apparatus is turned on. First, the diskdiscrimination device 12 discriminates the type of the optical disk.When it turns out that the optical disk has a single recording layer(the first optical disk 71), the aberration correction quantityswitching device 14 selects the aberration correction quantity (a)according to the instructions of the disk discrimination signal 13. Thequantity (a) is used to correct a spherical aberration that correspondsto a base material thickness error of 0 μm relative to the referencedisk. Then, the output signal (i.e., the first quantity of sphericalaberration correction) from the aberration correction quantity switchingdevice 14 is input to the adder 17. Moreover, the output signal (i.e.,the third quantity of spherical aberration correction) from thegravitational displacement correction quantity storage device 18 also isinput to the adder 17. The adder 17 adds the first and third quantitiesand outputs the result to the aberration correcting element drivingcircuit 8. The aberration correcting lens group 201 corrects sphericalaberration while considering the spherical aberration resulting from achange in the space between its lens groups due to gravity. Therefore, astable focus error signal can be provided in focusing control that isperformed on the recording layer after the correction.

Next, the correction of spherical aberration for a two-layer disk willbe described.

When the optical disk having two recording layers (the second opticaldisk 75) is discriminated by the disk discrimination device 12 andfocusing control is performed on the L0 layer 77, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (b) according to the instructions of the diskdiscrimination signal 13. The quantity (b) is used to correct aspherical aberration that corresponds to a base material thickness errorof 10 μm by which the base material of the optical disk is thinner thanthat of the reference disk. Then, the output signal (i.e., the firstquantity of spherical aberration correction) from the aberrationcorrection quantity switching device 14 is input to the adder 17.Moreover, the output signal (i.e., the third quantity of sphericalaberration correction) from the gravitational displacement correctionquantity storage device 18 also is input to the adder 17. The adder 17adds the first and third quantities and outputs the result to theaberration correcting element driving circuit 8. The aberrationcorrecting lens group 201 corrects spherical aberration whileconsidering the spherical aberration resulting from a change in thespace between its lens groups due to gravity. Therefore, a stable focuserror signal can be provided in focusing control that is performed onthe L0 layer 77 after the correction.

Similarly, when the optical disk having two recording layers (the secondoptical disk 75) is discriminated by the disk discrimination device 12and focusing control is performed on the L1 layer 79, the aberrationcorrection quantity switching device 14 selects the aberrationcorrection quantity (c) according to the instructions of the diskdiscrimination signal 13. The quantity (c) is used to correct aspherical aberration that corresponds to a base material thickness errorof 10 μm by which the base material of the optical disk is thicker thanthat of the reference disk. Then, the output signal (i.e., the firstquantity of spherical aberration correction) from the aberrationcorrection quantity switching device 14 is input to the adder 17.Moreover, the output signal (i.e., the third quantity of sphericalaberration correction) from the gravitational displacement correctionquantity storage device 18 also is input to the adder 17. The adder 17adds the first and third quantities and outputs the result to theaberration correcting element driving circuit 8. The aberrationcorrecting lens group 201 corrects spherical aberration whileconsidering the spherical aberration resulting from a change in thespace between its lens groups due to gravity. Therefore, a stable focuserror signal can be provided in focusing control that is performed onthe L1 layer 79 after the correction.

As the gravitational displacement correction quantity storage device 18,any means can be used as long as it stores the third quantity ofspherical aberration correction, e.g., a variable resistor, a flashmemory or EEPROM, and the same effect can be obtained.

As described above, an optical disk apparatus of this embodiment startsthe correction of spherical aberration before operating focusingcontrol. The first quantity of spherical aberration correction ispredetermined for each recording plane of an optical disk that issubjected to focusing control, among which an appropriate quantity isselected and switched in accordance with the type of the disk and thetarget recording plane. The third quantity of spherical aberrationcorrection is set while considering the spherical aberration resultingfrom a change in the space between the lens groups of the aberrationcorrecting lens group 201 due to gravity. Using the quantity ofspherical aberration correction obtained by adding the first and thirdquantities, the correction of spherical aberration for the recordingplane to be subjected to focusing control is performed before operatingthe focusing control. Thus, a favorable focus error signal can beprovided in the subsequent focusing control, so that the effect ofoperating stable focusing control can be achieved.

In the above configuration, the aberration correcting element drivingcircuit 8 drives the aberration correcting lens group 201 using theresult of addition of the first quantity of spherical aberrationcorrection from the aberration correction quantity switching device 14and the third quantity of spherical aberration correction from thegravitational displacement correction quantity storage device 18.However, the present invention is not limited thereto. For example, thereference value storage device 16 in Embodiment 7 can be used so thatthe aberration correcting lens group 201 is driven based on the quantityof spherical aberration that is obtained by adding the second quantityof spherical aberration correction from the reference value storagedevice 16 to the above result. Alternately, the following configurationalso can be employed, in which the disk discrimination device 12 and theaberration correction quantity switching device 14 are removed from theconfiguration shown in FIG. 12 and the reference value storage device 16in Embodiment 6 is used instead, so that the aberration correcting lensgroup 201 is driven based on the result of addition of the secondquantity of spherical aberration correction from the reference valuestorage device 16 and the third quantity of spherical aberrationcorrection from the gravitational displacement correction quantitystorage device 18.

As the method for discriminating the type of an optical disk with thedisk discrimination device 12 in this embodiment, any method can be usedas long as it distinguishes the type of an optical disk, e.g., a methodin which discrimination is made by detecting a hole formed in acartridge for housing an optical disk, by the shape of the cartridge, orby using the quantity of light reflected from the optical disk todistinguish between a single-layer disk and a two-layer disk.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1-16. (canceled)
 17. An optical disk apparatus comprising: an opticalpickup comprising a laser source, a focusing optical system forreceiving a light beam emitted from the laser source and focusing thelight beam on an optical disk to form a tiny spot, a transfer system fortransferring the focusing optical system in a direction substantiallyperpendicular to the optical disk, a photodetector for receiving a lightbeam reflected from the optical disk and outputting an electric signalin accordance with a quantity of light, and an aberration correctingsystem for correcting spherical aberration of the focusing opticalsystem; a focusing error detection device for detecting a focused stateof the tiny spot on the optical disk based on the output signal from thephotodetector; a focusing control device for controlling the focusedstate of the tiny spot on the optical disk so as to be a predeterminedstate by driving the transfer system based on an output signal from thefocusing error detection device; a disk discrimination device fordiscriminating a type of the optical disk; a reference value storagedevice for storing at least one quantity of spherical aberrationcorrection of the aberration correcting system with respect to at leastone optical disk having at least one reference thickness, and anaberration correction quantity switching device for switching a quantityof spherical aberration correction of the aberration correcting systemselectively based on a signal from the disk discrimination device,wherein the aberration correction quantity switching device presets thequantity of spherical aberration correction of the aberration correctingsystem based on an output signal from the reference value storage devicebefore operating the focusing control device.
 18. The optical diskapparatus according to claim 17, wherein the quantity of sphericalaberration correction of the aberration correcting system is determinedbased on a standard thickness of an intermediate layer of a two-layerdisk.
 19. An optical disk apparatus comprising: an optical pickupcomprising a laser source, a focusing optical system for receiving alight beam emitted from the laser source and focusing the light beam onan optical disk to form a tiny spot, a transfer system for transferringthe focusing optical system in a direction substantially perpendicularto the optical disk, a photodetector for receiving a light beamreflected from the optical disk and outputting an electric signal inaccordance with a quantity of light, and an aberration correcting systemfor correcting spherical aberration of the focusing optical system; afocusing error detection device for detecting a focused state of thetiny spot on the optical disk based on the output signal from thephotodetector; a focusing control device for controlling the focusedstate of the tiny spot on the optical disk so as to be a predeterminedstate by driving the transfer system based on an output signal from thefocusing error detection device; a disk discrimination device fordiscriminating a type of the optical disk; an aberration correctionquantity switching device for switching a first quantity of sphericalaberration correction of the aberration correcting system selectivelybased on a signal from the disk discrimination device; a reference valuestorage device for storing a second quantity of spherical aberrationcorrection of the aberration correcting system with respect to anoptical disk having a reference thickness; and an adder for adding thefirst and second quantities of spherical aberration correction, whereina quantity of spherical aberration correction of the aberrationcorrecting system is preset based on an output signal from the adderbefore operating the focusing control device.
 20. An optical diskapparatus comprising: an optical pickup comprising a laser source, afocusing optical system for receiving a light beam emitted from thelaser source and focusing the light beam on an optical disk to form atiny spot, a transfer system for transferring the focusing opticalsystem in a direction substantially perpendicular to the optical disk, aphotodetector for receiving a light beam reflected from the optical diskand outputting an electric signal in accordance with a quantity oflight, and an aberration correcting system for correcting sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; and a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the transfersystem based on an output signal from the focusing error detectiondevice, wherein a learning operation for spherical aberration correctionquantity is performed before operating the focusing control device, thelearning operation comprising steps of obtaining a first amplitude ofthe output signal from the focusing error detection device, storing thefirst amplitude, obtaining a second amplitude of the output signal fromthe focusing error detection device after changing a quantity ofspherical aberration correction of the aberration correcting system, andcomparing the first amplitude with the second amplitude.
 21. An opticaldisk apparatus comprising: an optical pickup comprising a laser source,a focusing optical system for receiving a light beam emitted from thelaser source and focusing the light beam on an optical disk to form atiny spot, a transfer system for transferring the focusing opticalsystem in a direction substantially perpendicular to the optical disk, aphotodetector for receiving a light beam reflected from the optical diskand outputting an electric signal in accordance with a quantity oflight, and an aberration correcting system for correcting sphericalaberration of the focusing optical system; a focusing error detectiondevice for detecting a focused state of the tiny spot on the opticaldisk based on the output signal from the photodetector; and a focusingcontrol device for controlling the focused state of the tiny spot on theoptical disk so as to be a predetermined state by driving the transfersystem based on an output signal from the focusing error detectiondevice, wherein a learning operation for spherical aberration correctionquantity is performed before operating the focusing control device, thelearning operation comprising steps of obtaining a first amplitude of areproduction signal, storing the first amplitude, obtaining a secondamplitude of the reproduction signal after changing a quantity ofspherical aberration correction of the aberration correcting system, andcomparing the first amplitude with the second amplitude.
 22. The opticaldisk apparatus according to claim 20, wherein the learning operation isperformed on every recording layer of the optical disk at the time theoptical disk is installed in the optical disk apparatus or at the timethe apparatus is turned on.
 23. The optical disk apparatus according toclaim 21, wherein the learning operation is performed on every recordinglayer of the optical disk at the time the optical disk is installed inthe optical disk apparatus or at the time the apparatus is turned on.24. The optical disk apparatus according to claim 17, wherein theaberration correcting system controls the quantity of sphericalaberration correction by changing a state of a light beam entering thefocusing optical system.
 25. The optical disk apparatus according toclaim 17, wherein the aberration correcting system controls the quantityof spherical aberration correction by changing a curvature of awavefront of a light beam entering the focusing optical system.
 26. Theoptical disk apparatus according to claim 17, wherein the at least onereference thickness includes 0.1 mm.
 27. The optical disk apparatusaccording to claim 17, wherein the at least one reference thicknessincludes 0.1 mm and a difference between a standard thickness of anintermediate layer of a two-layer disk and 0.1 mm.